The present invention relates to an apparatus and method for measuring the movement of a tall structure, particularly an offshore platform used for the production of hydrocarbons in water covered areas. Offshore platforms installed in some producing areas of the world are subject to extraordinary forces that are not present in other areas of the world. For example, platforms installed in the Cook Inlet of Alaska or offshore on the North Slope of Alaska are subject to large forces caused by the breakup of the ice each year. Also, platforms installed offshore California are subject to earthquakes which cause severe movements of the platform. Further, as the water depths in which petroleum operations are conducted increase, the height of the platform in relation to its overall cross sectional area becomes greater and the platform is subject to greater movement.
In order to confirm the design of offshore structures, attempts have been made using accelerometers to measure the movement of the platforms. The accelerometers measure the motion of the platform and these measurements can be used in determining whether the platform motion and the resulting stress is within the assumptions made during the design of the platform. While accelerometers have been used to measure platform movements, they are normally fixed to the above-water portion of the platform and only reflect the movement of that portion of the platform. Attempts have been made to place accelerometers on the supporting legs of platforms, either by permanently fastening the accelerometer package to the platform legs or lowering them into suitable tubular structures which have been affixed to the platform legs. While these systems have been successful in obtaining data relating to the movement of the platform, they do not present data taken on all the various legs of the platform simultaneously. It is desirable to make all measurements at the same time so that the natural frequency and mode shapes of the platforms can be calculated. While the use of accelerometer packages fastened to the legs of the platform are useful, they are unreliable due to the nature of the accelerometers and the inability to retrieve the package to effect repairs. Also, accelerometer packages that have been lowered into tubular members affixed to the legs of the platform provide measurements at only one position on the leg and not multiple positions which would be desirable.